In modern consumer electronics devices it is frequently necessary for various components, having in some cases only very low strut widths to be bonded to one another. In this case, the bonding strengths of pressure-sensitive adhesive tapes are often insufficient, hence making it necessary in such cases oftentimes to fall back on reactive liquid adhesives. The use of liquid adhesives, however, entails certain disadvantages, being linked as it is with considerable odor nuisance. Moreover, liquid adhesives are awkward to handle. Consequently, adhesive systems are sought that have a low vapor pressure and are available in substantially two-dimensional form for neater processing. One option is that of heat-activatable films (HAF). Since the substrates to be bonded to one another include heat-sensitive substrates, the requirement exists to offer HAF which can be processed even at very low temperatures (T<100° C.).
A type of adhesive bond for which demand has developed increasingly in recent years particularly in the electronics segment—for example, in cellphones or so-called laptops—is the adhesive bonding of eloxed aluminum and plastic. Eloxed aluminum is becoming ever more important as a decorative material for electronic devices. So-called eloxed or else anodized aluminum, i.e., aluminum processed using the Eloxal process, is aluminum on whose surface an oxidic protective layer has been formed by anodic oxidation of the aluminum. Here, in contrast to the electroplating techniques, the protective layer is not deposited on the workpiece; instead, an oxide or hydroxide is formed by conversion of the topmost layer of metal. It therefore has a particularly good connection to the aluminum. A layer 5 to 25 μm thick is formed, which protects underlying layers from corrosion for as long as no gaps are formed in this layer, as a result of mechanical damage, for example. A disadvantage of eloxed aluminum is its thermal sensitivity. Because the aluminum oxide has an expansion coefficient different from that of the aluminum itself, and is brittle, the layer of aluminum oxide, under the effect of heat even at temperatures around 100° C., develops stress cracks—in other words, the abovementioned unwanted gaps are produced in the protective layer. Thermal stressing even in this temperature range, moreover, results in visual defects, an unwanted phenomenon for decorative elements.
For the bonding strength on a substrate, it is particularly the chemical and physical surface properties that are critical in relation to the substrate. As far as the anchoring of an adhesive layer on its surface is concerned, therefore, eloxed aluminum is more like a ceramic (aluminum oxide groups and aluminum hydroxide groups) than a metal, with the specific challenges associated with the anchoring of adhesive layers on ceramic, and tends to present fewer of the challenges associated with the anchoring of adhesive layers on metals.
The hot pressing of HAF diecuts is now established practice and is known, for example, for the adhesive bonding of chip modules into chip cards. The hot adhesive bonding of polycarbonate cell phone casings to a decorative aluminum section is described in the Applicant's DE 10 2005 035 905 A1 (=U.S. Pat. No. 7,923,510). Here, pressing temperatures of 180° C. are employed. Eloxed aluminum is too heat-sensitive for bonding temperatures of 180° C., and is therefore also not explicitly stated in the specification.
The Applicant's DE 10 2009 006 935 A1(=U.S. 2012/027986) describes heat-activatable films comprising a nonwoven carrier for the bonding of metals and plastics. Among the substrates listed is anodized (eloxed) aluminum. Preference is given to using thermoplastic materials having a melting temperature of more than 85° C. Even this temperature, however, is too high for the bonding of eloxed aluminum. Thermoplastic polyurethanes are stated, though not explicitly those obtained from aqueous dispersion. Although isocyanates can be employed as reactive resins, no particular treatment of the isocyanates is stated. This reactive-resin component is also mentioned as a component for addition to the thermoplastic polyurethane; the thermoplastic matrix itself is not a reactive system.
WO 93/25599 A1 (Thomas Abend) discloses formulations for latently reactive polyurethane systems that comprise deactivated polyisocyanates, having reactivity at temperatures above 55° C., and comprising polymers which at temperatures above 40° C. are meltable and are able to react with isocyanate. These formulations may be coated onto substrates, which can then be bonded. They may also be shaped in the form of a carrier-free film, which can then be laminated to a substrate to be bonded. Adhesive systems of this kind serve for the joining, sealing, laminating, or coating of wood, plastics, metals, glass, textiles, synthetic sheetlike structures, card, paper, and foils. These materials are not defined in any more detail. There is no mention of a plastic/metal combination. Nor is there any reference to eloxed aluminum.
J. Büchner, W. Henning, Adhäsion, 2007, 51(6), 16-21 describes drying processes for aqueous, latently reactive polyurethane dispersions, and also pressing approaches for the adhesive bonding of two substrates. Example substrates cited are PVC, leather, and MDF (medium-density wood fiberboard). There is no mention of adhesive bonding of metal/plastic combinations, and eloxed aluminum is not recited.
DE 10 2010 013 145 A1 (Lohmann) describes adhesives which are heat-activatable and latently reactive. At room temperature they have slight tack and still exhibit adhesiveness for a certain time following initial heating and cooling. These adhesives are based preferably on polyurethanes. Substrates stated for bonding include metals and plastic, without further particularization, and is there is particular reference to the application of bonding heat-sensitive plastics in the electronics industry. A plastic/metal combination is not stated; eloxed aluminum is not mentioned.
WO 99/29755 (=U.S. Pat. No. 6,348,548) (Thomas Abend) describes reactive polyurethane adhesive systems based on aqueous polyurethane dispersions. Dispersed into the matrix of a thermoplastic polyurethane, which still carries functional groups for reaction with isocyanates, are polyisocyanate particles that are deactivated on their surface. At a first temperature, the thermoplastic polyurethane melts. At a temperature above this, the deactivated particle surface dissolves and the isocyanate groups are able to react with the functional groups of the thermoplastic polyurethane. Substrates said to be suitable for the adhesive bonding are, generally, metals, plastics, glass, wood, wood composites, card, films/foils, synthetic sheetlike structures, and textiles. The possibility of combination of materials is not stated, and nor is any reference made to eloxed aluminum.
None of the stated texts offers a solution for the adhesive bonding of eloxed aluminum to plastic. The search continues, therefore, for a suitable means of adhesive bonding of eloxed aluminum to plastic, that can be provided in substantially two-dimensional form, that can be applied to a substrate (eloxed aluminum or plastic) at a low temperature above room temperature, i.e., at a temperature even below 85° C., and that in technical terms passes the so-called push-out test after thermal pressing to the second substrate (plastic or eloxed aluminum), and achieves this not only before but also after specific humidity/heat storage. These requirements are typical for the qualification of adhesive bonds in the consumer electronics segment.
The bonding of a polar plastic (e.g., polycarbonate or PMMA) to a moisture-impermeable substrate (in the present case, eloxed aluminum) harbors special difficulties. Composite laminates of this kind are severely stressed in humidity/heat storage, since the polar plastic is able to take up a considerable quantity of moisture from the environment. This moisture presses on the adhesive film surface that is in contact with the plastics substrate. To a certain degree, the adhesive film is able to take up moisture from the plastic and emit it to the environment again. In a plastic/eloxed aluminum bond, however, this moisture is unable to escape over the full surface area, instead being able to escape only via the end faces, because the adhesive film is backed by a moisture-impermeable substrate (eloxed aluminum). The risk exists of the adhesive film detaching locally and partially from the plastics surface—an unwanted phenomenon. It may therefore be concluded that an appropriate means of adhesive bonding is an adhesive which itself has a fairly low moisture content and is not obtained from water-based dispersion, with which, in spite of drying, the possibility exists of substantial residual moisture.
It has been found, moreover, that in the case of metals and particularly for eloxed aluminum, moisture may tend to creep between the surface of the eloxed aluminum, in other words the typically 5 μm to 25 μm thick ceramic layer, and the surface of the adhesive film, possibly leading to failure of the otherwise good bonding performance. The bonding of eloxed aluminum to plastic therefore imposes a particularly exacting requirement on any bonding solution that is to be found. Bonding means suitable for the aluminum metal/plastic system cannot necessarily be employed, since the aluminum oxide of the surface layer on the eloxed aluminum has entirely different properties from the metallic aluminum, being, for example, much more brittle than the latter.